1. Field of the Invention
The present invention relates to a stabilized composition of an inorganic filler-containing polyolefin. More particularly, the present invention is concerned with a polyolefin resin composition stabilized against accelerated deterioration attributable to the incorporation therein of an inorganic filler.
2. Description of the Prior Art
Polyolefin resins, such as polyethylene, polypropylene, polybutene-1 or poly-4-methylpentene, have excellent mechanical properties and have been used as the materials, of manufacture for various molded articles, films and fibers. Various antioxidants, such as phenolic, thioether, phosphite or phosphonite antioxidants, have hitherto been used for the purpose of preventing thermal oxidative deterioration of the above-described polyolefin resins.
In particular, when the above-described polyolefin resins are used for manufacturing various molded articles, for example, automotive parts and electrical components, inorganic fillers, such as talc or calcium carbonate, are often incorporated in the polyolefin resins for the purpose of improving tensile strength, bending strength, rigidity, thermal deformation temperature, dimensional stability, etc. It is known, however, that these inorganic filler-containing polyolefin resins are remarkably inferior in mechanical properties, thermal stability, etc., to the polyolefin resin free from any inorganic filler.
This is thought to be due to poor affinity of the inorganic filler for the polyolefin resin. For this reason, treatment of the surface of the inorganic filler with a fatty acid ester of a polyhydric alcohol or the like, or the addition of this fatty acid ester or the like in combination with the inorganic filler has been disclosed in, for example, Japanese Patent Laid-Open Nos. 49254/1977 and 80345/1977. However, the lowering of thermal stability could not be prevented through the mere treatment of the inorganic filler with the above-described compound.
It is further believed that the lowering of thermal stability is due to the influence of minute amounts of heavy metals contained in the inorganic filler, or copper, iron, etc., with which the polyolefin comes into contact when used as parts of industrial machines. In order to inhibit the unfavorable action caused by the above-described heavy metals, metal inactivating agents, for example, melamine, guanamine, benzoguanamine, 3-salicyloylaminotriazole, N,N'-dibenzylidene(oxalic dihydrazide), N-salicyloyl-N'-salicylidenehydrazine, bissalicyloylhydrazine, bis[.beta.-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine, dodecanedioic acid bis(salicyloylhydrazide) and bis[.beta.-(3,5-di-tert-butyl4-hydroxyphenyl)propionyloxyethyl]oxamide, have been used in combination with various antioxidants.
Although the combined use of an antioxidant and a metal inactivating agent results in an improvement to some extent when no inorganic filler is incorporated in the polyolefin resin, the effect is quite insufficient when inorganic filler is incorporated, so that the combination is far from being satisfactory from the viewpoint of practical use.
U.S. Pat. No. 3,553,158 has suggested that the lowering of thermal stability caused by the above-described incorporation of the inorganic filler is due to the adsorption of the phenolic antioxidant on the inorganic filler used and proposed the use of the phenolic antioxidant in combination with compounds having a polar group, such as an epoxy compound, an amide compound, an ether compound, a sulfide compound, a hydroxy compound or an ester compound for the purpose of preventing the adsorption. Based on a similar concept, Japanese Patent Publication No. 50981/1980 has proposed the use of an antioxidant comprising a hindered phenol ester of isocyanuric acid because the polyolefin resin is less susceptible to a decrease in thermal stability caused by the incorporation of the inorganic filler, in combination with an ester of thiodipropionic acid and the above-described compound having a polar group. Japanese Patent Laid-Open No. 188543/1989 has proposed the use of an epoxy compound in addition to the combination of an antioxidant and a metal inactivating agent; and Japanese Patent Laid-Open No. 282232/1989 has proposed the use of a particular phenolic antioxidant in combination with an amide compound.
In the methods described in the above-described publications, although the thermal stability is improved to some extent, the effect is insufficient and, further, the coloration of the resin composition is often significantly unfavorable, so that prior art methods are unsatisfactory from the viewpoint of practical use.